U.S. President Barack Obama has spoken by telephone with Russian counterpart Vladimir Putin, with Obama saying the United States remains deeply concerned about Moscow's increased support for separatists in Ukraine.

Speaking to reporters at the White House Friday, President Obama said he discussed his preference for a diplomatic solution with the Russian leader. Mr. Obama, however, said there are limits to what the United States can do.

"Right now what we've done is impose sufficient costs on Russia that objectively speaking they should, President Putin should want to resolve this diplomatically. Get these sanctions lifted. Get their economy growing again and have good relations with Ukraine; but sometimes people don't always act rationally and they don't always act based on their medium- or long-term interests. That can't deter us though. We just have to stay at it," said President Obama.

The White House says the two leaders "agreed to keep open their channels of communication."

The Kremlin, for its part, said Putin told the U.S. president in the telephone call that fresh sanctions imposed on Russia this week for its support for the separatists were "counterproductive" and would cause "serious damage to bilateral cooperation and international stability."

Also Friday, several dozen international investigators began working at the crash site of Malaysia Airlines Flight 17 in eastern Ukraine.

Investigators from the Netherlands and Australia, along with officials from the Organization for Security and Cooperation in Europe, or OSCE, began combing an area now designated as a crime scene.

They will focus on recovering several dozen bodies still missing and retrieving the belongings of the 298 people killed when the Boeing 777 was shot down last month.

In another development, 10 Ukrainian paratroopers were killed Friday in an ambush by pro-Russian separatists near the town of Shakhtarsk, which is located not far from the crash site.

The acting commander of Ukraine's airborne troops, Colonel Yuriy Halushkin told reporters in Kyiv 13 other paratroopers were wounded and 11 are missing.

Ukraine and Western governments blame rebels for the shootdown. U.S. analysts say the jetliner was likely downed by pro-Russian separatists launching a Russian missile. There were no survivors.

Rebels intent on establishing autonomous republics near the Russian border have been battling Ukrainian troops for three months.